The present invention relates to a gas laser apparatus, and particularly to the technique for facilitating the maintenance of a transparent window provided as a laser output portion in a laser chamber body in which gas is sealed.
Recently, attention has been paid to a gas laser as a light source for various industries. Particularly, a krypton fluoride excimer laser and an argon fluoride excimer laser have been expected to be applied to fields of a semiconductor lithography light source and the like as a powerful ultraviolet light source, and the study thereof toward the practical use has been progressed.
The excimer laser is called a discharge exciting type, in which two electrodes are arranged in an atmosphere of a mixture of rare gases such as xenon, krypton and argon and halogen gases such as chlorine and fluorine, and a high voltage discharge is produced between the electrodes to thereby constitute an optically active dimer.
This dimer serves as a laser medium to obtain the ultraviolet laser oscillation of 248 nm in the krypton fluoride excimer laser type and 193 nm in the argon fluoride excimer laser type.
The aforesaid gas mixture is filled in a pressure vessel called a laser chamber, which is provided with discharge electrodes made of aluminum nickel or other metal and a gas circulating device, and a beam removing laser window formed of nitric material having an excellent ultraviolet permeative characteristic. Externally of the laser chamber are arranged a high voltage power source, a gas supply system, various control systems and so on.
A voltage of 20 to 30 KV is applied between the aforementioned discharge electrodes to create discharges at high frequency. It is known that the discharge electrodes become worn due to the sputtering effect during that period.
Metal powders generated due to the aforesaid abrasion are formed into powdery dusts, which float within the laser chamber and a part thereof becomes deposited on the laser window. When an ultraviolet laser beam is irradiated on surface of the laser window.
If such a state as described above is present, a laser output to be removed outside passing through the laser window is not only lowered but also the stable laser output cannot be obtained.
To cope with this situation, in prior art, gases are circulated within the laser chamber to blow the gases against the internal surface of the laser window so as to blow off the powdery dust close to the laser window.
Furthermore, in the case where the aforesaid technique is insufficient, a laser window 51 provided in a laser chamber 50 is designed to be detachable as shown in FIG. 8, so that the laser window 51 is periodically replaced to maintain the transparency.
However, in the aforesaid conventional apparatus, gases containing powdery dust are merely circulated to blow them against the internal surface of the laser window, and therefore, there is a problem in that the effect for preventing adhesion of powdery dust is low.
Another problem is that even in the case of the construction in which the laser window can be replaced as described above, when replacing, the interior of the laser chamber 50 comes into communication with open air, and as a result a gas leak and entry of outside air tend to occur.
Since the outside air contains oxygen or water content, these cause the interior of the laser chamber to be stained, thus requiring the running-in for restoration.
As described above, much labor and cost are required for replacement of a laser window and thereafter maintenance.
The present invention has been achieved in view of the aforementioned circumstances and provides the technique which can keep the interior of a laser chamber clean for long periods and which can prevent the interior of the laser chamber from contracting outside air when the laser window is replaced despite a handy construction.